Method of electrophoretically covering helically wound heating wires with an insulating layer

ABSTRACT

A helically wound V-shaped bent reciting wire or bifiller wound heating wire for an indirectly heated cathode is previously dipped in a liquid or connected anaphoretically and then covered in an electrophoresis bath with an insulating layer.

Feb. 26, 1974 R. G. H. COENEN ETAL 3,794,593

- METHOD OF ELECTRCPHORETICALLY COVERING HELICALLY WOUND HEATING WIRESWITH AN INSULATING LAYER Filed Aug. 17, 1972 United States Patent METHODOF ELECTROPHORETICALLY COVER- ING HELICALLY WOUND HEATING WIRES WITH ANINSULATING LAYER Renrer Gertruda Hubertus Coenen, Adrianus Kuiper, andMaria Antonius Alphonsus Andreas Collaris, Emmasingel, Eindhoven,Netherlands, assignors to US. Philips Corporation, New York, N.Y.

Filed Aug. 17, 1972, Ser. No. 281,345 Claims priority,applicati7olrillggaherlands, Aug. 18, 1971,

Int. Cl. B01k /02; C23b 13/00 US. Cl. 204-181 Claim ABSTRACT OF THEDISCLOSURE A helically wound V-shaped bent heating wire or bifilarwoundheating wire for an indirectly heated cathode is previously dipped in aliquid or connected anaphoretically and then covered in anelectrophoresis bath with an insulating layer.

The invention relates to a method of electrophoretically coveringhelically wound heating wires for indirectly heated cathodes with aninsulating layer, in particular heating Wires which are bent in the formof a V once or several times or bifilar-wound wires. The inventionfurthermore relates to a heating wire comprising an insulating layerprovided according to said method.

Heating wires for indirectly heated cathodes are usually coveredelectrophoretically with an insulating layer so that the ends of thehelically wound heating wire which is usually bent in the form of a Vonce or several times or is a bifilar-wound wire, remain free frominsulating material. The wire is usually connected to current supplywires by welding with the bare ends.

With the present-day required accuracy it is of importance that theseparation between the covered part and the non-covered ends of theheating wire be accurately determined and the separation line be at thesame distance from the ends of all the heating wires so as to preventinsulation material from reaching the places of the wire to be welded.It has been found that comparatively large tolerances have to bepermitted for the place of said separation since rather large deviationsof the desirable place occur.

Experiments have proved that this drawback is caused in that thehelically wound wire has capillary spaces so that the suspensiondestined for electrophoresis is drawnin by capillary action into theends of the helically wound heating wire not dipped in the suspension.

According to the invention, measures are taken to reduce the influenceof the capillary action in the place of separation between covered andnon-covered parts. This can be done in a simple manner by filling thespaces of the wound wire with a suitable liquid just prior to dippingthe Wire in the electrophoresis bath. Since the capillary spaces arefilled by the said liquid, no suspension of insulating material of theelectrophoresis bath is drawnin any longer. As a dipping liquid ispreferably used the same liquid as that in which the insulating materialis suspended.

According to another method, the wound wire may be dipped in theelectrophoresis bath and the electrophoresis voltage be applied rapidlyas soon as the wire is dipped to the correct depth. The electric fieldwhich moves the insulating particles in the suspension to the wire in adirection normal to the wire then prevents the particles from beingtaken along with the capillary flow parallel to the axis of the woundwire. Moreover, the gaps between the turns of the wire rapidly slit upso that mainly only suspension liquid is drawn above the dipping levelby 3,794,593 Patented Feb. 26, 1974 capillary action. If desirable thewire may be kept at an anapheretic potential during dipping the wire inthe suspension, after which the voltage is switched in the desireddirection for cataphoresis after the wire has been dipped to the correctdepth in the electrophoresis bath. During dipping of the wire, theinsulating particles are repelled so that substantially only the liquidis drawn in the capillary spaces. Only then is the potential switched.

The invention enables the usual tolerances for the spot of theseparation of .covered and uncovered parts to be reduced to A.

The invention will be described in greater detail with reference to thedrawing, of which:

FIG. 1 shows diagrammatically an arrangement for carrying out the methodaccording to the invention, while FIGS. 2 and 3 show embodiments ofheating wires covered by said method.

Reference numeral 1 in FIG. 1 denotes a shaped heating wire which iswound helically and bent to form a V. The helically wound wire may alsobe bent back and be a bifilar-wound wire. The wire 1 is clamped in aholder 2. The part of the shaped wire 1 present between the tip of the Vand the line AA is to be coated electrophoretically with insulatingmaterial. If, as usual, the wire 1 is dipped to the line AA in anelectrophoresis bath 4 which is present in a container 3, the suspension4 will creep up the helically wound wire to slightly above the line AAas a result of the capillary action and reach the ends 9 of the wire,which ends, however, have to remain bare. So during electrophoresis asmall quantity of insulating particles will also deposit on the ends 9which impedes the welding of said ends to current supply conductors.

It has moreover been found that the distance over which the suspensioncreeps up by capillary action is not constant so that proportionallylarge tolerances for the place of the separation between the covered andthe uncovered parts of the heating wire 9 have to be permitted.

This drawback is considerably reduced by .the various measures which aretaken according to the invention in the method of electrophoreticallycovering such heating wires.

A very suitable measure is that in which the heating wire 1 is dipped ina suitable liquid down to the holder 2, said liquid filling thecapillary spaces. Right after this, the wire is dipped in the bath 4.The suspension can now no longer creep up. The dipping liquid ispreferably the same liquid as that in which the insulating material ofthe suspension 4 is suspended.

The suspension 4 preferably consists of aluminium oxide powder which issuspended in methyl alcohol. The liquid being not electricallyconductive, covering of the filament is carried out in this case bycataphoresis, the negative terminal of the current source 5 beingconnectable to the holder 2 by means of a switch 6. The container 3 isconnected to a positive potential. It is possible to apply the negativepotential to the holder 2 only after the heating wire 2 has been dippedin the suspension 4 down to the line AA. In that case a uniformly thickinsulating layer 7 is obtained as is shown in FIG. 2. It is ofadvantage, however, to give the tip of the V a thickening 8 and to makethe thickness of the remaining insulating layer 7 thin. The thickening 8serves to center the heating member in a cathode support, while thelayer 7 does not contact the cathode support. A thin layer 7 has theadvantage of being slightly flexible so that the V- shaped parts can besqueezed a little when inserted into a cylindrical support of a cathodewithout the layer 7 chipping oflf.

In order to obtain such a thickening 8 the cataphoresis potential isalready applied before the V-shaped helically the deposition ofinsulation material on the ends 9 can also be avoided by applying ananaphoretic potential at the wire 1 during the dipping of the wire 1 inthe suspension 4. The holder 2 with the Wire 1 is then made positiverelative to the suspension 4. The insulating particles from thesuspension which are drawn in the wire turn 1 by capillary action arethen repelled so that substantially only the suspension medium can reachthe capillary spaces. When, after dipping the wire 1 down to theline AA,said liquid has reached its largest height above the line AA, the switch7 is switched so that the wire 1 becomes negative relative to thesuspension 4- and insulating material deposits on the wire 1 down to theline AA. A thickening 8 can be obtained in this case by keeping the tipof the shaped wire in the suspension for some time when the wire 1 isdrawn out of the suspension.

Although a single embodiment'of a device for electrophoreticallycovering articles has been described, it is obvious that other knownelectrophoresis devices may also be used. For example, the potentialsmay be applied to separate electrodes placed in the suspension 4.

What is claimed is:

1. A method of electrophoretically covering a helically wound heatingwire for indirectly heated cathodes with an insulating layer to a givendistance from the ends of the wire, which ends are clamped in a holderduring covering, comprising the steps of dipping the heating wire downto the desirable depth into the electrophoresis bath, applying ananaphoretic potential to the wire until the liquid in the capillaryspaces has-reached its highest position, and thereafter, applying theelectrophoresis potential.

References Cited UNITED STATES PATENTS 3,049,482 8/19 62 Dolan 204181

